1. Field of the Invention
The field of the invention is that of optical devices for detecting the instantaneous position and instantaneous orientation of the helmet worn by an aircraft pilot. In a general manner, a particular position and orientation of the helmet will be called a posture hereinafter in the text. For certain aeronautical applications, pilots' helmets are furnished with viewing devices making it possible to generate, in the pilot's field of view, information relating to piloting, to navigation or to the weapons system. These helmet visual displays are generally coupled to helmet position and orientation detection systems.
2. Description of the Prior Art
There exist various systems making it possible to pinpoint the position of a helmet. Systems relying on the analysis of optical signals representative of the position of the helmet are used in particular. These systems necessarily comprise one or more sources of luminous emission and one or more sources of luminous reception. The emission sources may be luminous sights or point sources of light-emitting diode type, fixed to the helmet in a particular arrangement, such as a triangle. The position of the helmet in a determined zone is then obtained by analysis of the images of the diodes received by cameras from several viewpoints and by geometric calculation; the position of the head in space is deduced. Such devices have been produced by the company Karl Zeiss/Denel. It is possible, conversely, to arrange linear arrays of photo-sensors or photodiodes on the helmet and to illuminate them by projectors of particular images in such a way that the analysis, either spatial or temporal, of the signals received by the various photo-detectors makes it possible to retrieve the helmet posture information.
Whatever procedure is chosen, the detected signal is disturbed by solar illumination. Indeed, a part of the solar illumination is diffused by the helmet toward the reconnaissance cameras. It is known that the solar illumination can reach 70 000 lux in the case of a cockpit canopy having a transmission of 70%. The detected signal becomes hardly utilizable if the solar illumination received by the helmet is significant. When the emission sources are on the helmet, their signal is drowned in solar illumination. When the photo-detectors are on the helmet, the signal received from the source is drowned in solar illumination. The means conventionally used to improve detection consists in providing significant source power. It is also possible to emit and receive in a wavelength span situated outside of the visible solar radiation, that is to say situated either in the infrared, or in the near ultra-violet. However, the levels of solar illumination are still high in the infrared and ultraviolet bands and this type of solution requires specific emission and reception sources which necessarily increase the costs of the detection system.
Another solution is represented in FIG. 1. It consists in arranging on the helmet 1, sights 3 represented by triangles in this figure and comprising at least one first optical element 5 of “catadiopter” type having a very high retro-reflection coefficient and a very low diffusion coefficient in the visible region. The detection device comprises a set of fixed cameras 2 associated with an image processing system. Thus, the solar radiation RS is necessarily returned toward the sun as may be seen in FIG. 1 and may not reach the detection cameras 2. For night operation, the detection device comprises a set of fixed light sources 6 illuminating the helmet, the catadiopter 5 having a very high retro-reflection coefficient and a very low diffusion coefficient in the region of emission of the source. Opto-mechanical means 61 and 62 making it possible to produce an image of the light source 6 on the optical axis of the camera 2 complete the device. By using several sights 3 appropriately distributed over the helmet and several fixed cameras, it is possible to cover the entire volume of sweep 4 of the helmet.
Generally, any optical reflector or retro-reflector having the property of reflecting a pencil of light in the same direction as its incident direction is called a “catadiopter”. There exist diverse optical means of achieving this function. The subsequent description is more particularly concerned with “optical cube corner” catadiopters. An “optical cube corner” 5 such as represented in FIG. 2 consists of three mutually orthogonal plane mirrors 51. Thus, a pencil of light emitted by the emitting part and illuminating the catadiopter 5 is re-emitted in the same direction toward the receiving part with excellent efficiency as seen in FIG. 3. This figure represents the propagation of light rays L issuing from a point source S in an optical cube corner 5. For the sake of clarity, in this figure, the propagation of the rays takes place in a plane parallel to one of the three mirrors constituting the cube. In this plane, the light rays undergo only two reflections on the mirrors 51. It is easily demonstrated that the image of the source S is a source S′ situated on an axis SC passing through the source S and the center C of the optical cube corner and at equal distance D from the latter. In the same manner, any light beam which does not issue from the source and which strikes the catadiopter produces, on principle, hardly any illumination directed toward the receiving part.
It is clear that the use of a single catadiopter is insufficient to achieve the helmet's instantaneous position and instantaneous orientation function. It may be shown that this reconnaissance is possible by using, inter alia, a tetrahedron whose four vertices are catadiopters. As an example, a method for recognizing the 2D projection of a tetrahedron is described in European patent EP 0 294 101 from the company EI-Op. In this patent, four sources distributed to the four vertices of a tetrahedron give four image points in the image plane of a camera. On the basis of the knowledge of the projected coordinates of the four points, the position of the tetrahedron in space is calculated, once the indeterminacies have been resolved by discrimination of the points and the elimination of aberrant configurations (four given points on a plane correspond to sixteen possible configurations of tetrahedrons of known geometry but whose orientation is to be determined).
Of course, the dependability of the relative position of the points has a direct impact on the accuracy of the position measurement. Indeed, it may be demonstrated that any defect in the geometry of the tetrahedron either from the outset or due to an alteration in the structure will not only degrade the measurement accuracy but also introduce new ambiguities in the discrimination of the points. As an example, an angular deviation of 0.5 mrad in the position of a point of the tetrahedron gives rise to an error of 1 mrad in the measurement. The rigidity of the support, which guarantees compliance with the relative dimensional coordinates of points, is therefore crucial. It is a definite handicap for systems which, being mounted on the user's head, must remain as lightweight as possible.